1. Field of the Invention
The present invention relates to ophthalmic lenses, and more particularly, to contact lenses designed to slow, retard or prevent myopia progression. The ophthalmic lenses of the present invention comprise asymmetric radial power profiles that increase the radial dioptric power from the center to the margin of the optical zone of the lenses for preventing and/or slowing myopia progression.
2. Discussion of the Related Art
Common conditions which lead to reduced visual acuity are myopia and hyperopia, for which corrective lenses in the form of spectacles, or rigid or soft contact lenses, are prescribed. The conditions are generally described as the imbalance between the length of the eye and the focus of the optical elements of the eye, myopic eyes focusing in front of the retinal plane and hyperopic eyes focusing behind the retinal plane. Myopia typically develops because the axial length of the eye grows to be longer than the focal length of the optical components of the eye, that is, the eye grows too long. Hyperopia typically develops because the axial length of the eye is too short compared with the focal length of the optical components of the eye, that is, the eye does not grow enough.
Myopia has a high prevalence rate in many regions of the world. Of greatest concern with this condition is its possible progression to high myopia, for example greater than five (5) diopters, which dramatically affects one's ability to function without optical aids. High myopia is also associated with an increased risk of retinal disease, cataract, and glaucoma.
Corrective lenses are used to alter the gross focus of the eye to render a clearer image at the retinal plane, by shifting the focus from in front of the plane to correct myopia, or from behind the plane to correct hyperopia, respectively. However, the corrective approach to the conditions does not address the cause of the condition, but is merely prosthetic or symptomatic.
Most eyes do not have simple myopia or hyperopia, but have myopic astigmatism or hyperopic astigmatism. Astigmatic errors of focus cause the image of a point source of light to form as two mutually perpendicular lines at different focal distances. In the foregoing discussion, the terms myopia and hyperopia are used to include simple myopia or myopic astigmatism and hyperopia and hyperopic astigmatism respectively.
Emmetropia describes the state of clear vision where an object at infinity is in relatively sharp focus with the eye lens relaxed. In normal or emmetropic adult eyes, light from both distant and close objects and passing though the central or paraxial region of the aperture or pupil is focused by the crystalline lens inside the eye close to the retinal plane where the inverted image is sensed. It is observed, however, that most normal eyes exhibit a positive longitudinal spherical aberration, generally in the region of about +0.5 Diopters (D) for a 5 mm aperture, meaning that rays passing through the aperture or pupil at its periphery are focused +0.5 D in front of the retinal plan when the eye is focused to infinity. As used herein the measure D is the dioptric power, defined as the reciprocal of the focal distance of a lens or optical system, in meters. Also as utilized herein, the term “add” shall be defined as an additional plus power to assist in seeing more clearly at near distances.
The spherical aberration of the normal eye is not constant. For example, accommodation, that is, the change in optical power of the eye derived primarily though change to the internal crystalline lens causes the spherical aberration to change from positive to negative.
Emmetropisation is the process whereby eye growth is self-regulated to achieve an optimum match between the optics and axial length of the eye. Emmetropisation is responsible for the leptokurtosis apparent in refractive error distribution in humans and has been demonstrated to act in various animals to compensate for visual deprivation induced refractive errors. Juvenile-onset myopia is a common form of refractive error beginning in childhood and progressing up until the mid to late teens.
Whilst the length of the eye increases throughout life, growth is most pronounced during childhood. It has been observed that spherical aberration of the eye changes with age in children (Stine, 1930; Jenkins, 1963), from negative spherical aberration in children younger than about 6 years of age when focused on distant objects, to positive spherical aberration at about 6-7 years of age. Most adults display positive spherical aberration of the eye focused at infinity for the remainder of their lives.
U.S. Pat. No. 6,045,578 discloses a method of altering the focus of the eye, including changing the spherical aberration of the ocular system by a direction and degree related to alter the growth in eye length, in other words emmetropisation may be regulated by spherical aberration. In this process, the cornea of a myopic eye is fitted with a lens having its outer surface formed having increasing dioptric power away from the lens center. Paraxial light rays entering the central portion of the lens are focused on the retina of the eye, producing a clear image of an object. Marginal light rays entering the peripheral portion of the cornea are focused in a plane between the cornea and the retina, and produce positive spherical aberration of the image on the latter. This positive spherical aberration produces a physiological effect on the eye which tends to inhibit growth of the eye, thus mitigating the tendency for the myopic eye to grow longer. The higher the spherical aberration, the more profound is the effect on myopia progression. However, increases in the magnitude of the effective add power of the contact lens tends to worsen foveal vision.
Accordingly, there exists a need to improve distance vision correction and/or create a larger effective add power in contact lenses while preventing and/or slowing myopia progression via the introduction of positive aberration while maintaining good foveal vision.